Method of removing carbon from nuclear fuel elements in a closed system

ABSTRACT

METHODS ARE DISCLOSED FOR ELIMINATING CARBON FROM NUCLEAR FUEL ELEMENTS IN THE REPROCESSING THEREOF IN A CLOSED SYSTEM. IN ONE EMBODIMENT, THE CARBON IS REACTED WITH CARBON DIOXIDE TO FORM CARBON MONOXIDE IN AN ATTACK ZONE WHICH IS MAINTAINED AT A TEMPERATURE OF ABOUT 1000*C., THE CARBON MONOXIDE IS THEN CAUSED TO FLOW TO A REGENERATION ZONE WHICH IS MAINTAINED AT A LOWER TEMPERATURE SO THAT A REVERSE REACTION OCCURS WHICH CONVERTS THE CARBON MONOXIDE INTO CARBON DIOXIDE AND REGENERATED CARBON, THE CARBON DIOXIDE FLOWS BACK TO THE ATTACK ZONE LEAVING THE REGENERATED CARBON BEHIND IN THE REGENERATION ZONE.

G. DOLCI ET METHOD OF REMOVING CARBON FROM NUCLEAR FUEL ELEMENTS IN ACLOSED SYSTEM Filed Jan. 26, 1970 3 Sheets-Sheet 1 INVENTORS Jan. 30,1973 DQLcl ETAL 3,74,323

METHOD OF REMOVING CARBON FROM NUCLEAR FUEL ELEMENTS IN A CLOSED SYSTEMFiled Jan. 26, 1970 3 Sheets-Sheet 2 INVENTOR.

Jan. 30, 1973. DQLCI ETAL 3,7l4,323

METHOD OF REMOVING CARBON FROM NUCLEAR FUEL ELEMENTS IN A CLOSED SYSTEMFiled Jan. 26, 1970 3 Sheets-Sheet .3

FIG. 3

INVENTOR. M M

United States Patent Otfice 3,714,323 Patented Jan. 30, 1973 3,714,323METHOD OF REMOVlN G CARBON FROM NUCLE- AR FUEL ELEMENTS IN A CLOSEDSYSTEM Gioacchino Dolci, Pisa, and Ruggero Renzoni, Milan, Italy,assignors to Snarn Progetti S.p.A., Milan, Italy Filed Jan. 26, 1970,Ser. No. 5,769 Claims priority, application Italy, Jan. 21, 1969,11,765/69 Int. Cl. C22b 61/04 U.S. Cl. 423-4 6 Claims ABSTRACT OF THEDISCLOSURE Methods are disclosed for eliminating carbon from nuclearfuel elements inthe reprocessing thereof in a closed system. In oneembodiment, the carbon is reacted with carbon dioxide to form carbonmonoxide in an attack zone which is maintained at a temperature of about1000 C., the carbon monoxide is then caused to flow to a regenerationzone which is maintained at a lower temperature so that a reversereaction occurs which converts the carbon monoxide into carbon dioxideand regenerated carbon, the carbon dioxide flows back to the attack zoneleaving the regenerated carbon behind in the regeneration zone.

The present invention relates to a process for removing carbon, in aclosed system, from a certain zone of said system to another one, bychanging only the temperature and pressure conditions.

Said process may be practiced by making use of suitable catalysts orwithout using them.

In the reprocessing of some nuclear fuels of HTGR types (hightemperature gas cooled reactors) it is necessary to separate largeamounts of carbon from the active material (U, Pu, Th) which is presentin the form of little spheres. This carbon constitutes the coating ofthe spheres, the matrix wherein they are dispersed and the structuralgraphite which constitute a real fuel element. The proportion by weightbetween active material and carbon varies from a minimum of A to andmore too, according to the possibility and convenience of eliminatingpart of structural graphite, before the effective reprocessing.

All the tested processes are subject to serious disadvantages whenpracticed on a large scale.

These processes may be briefly described as follows:

(A) Grinding of the whole element to obtain grains smaller than thediameter of the particles. Acid attack of the powder for recovering theactive elements to be decontaminated.

(B) Combustion of the whole or ground element (fluidized bed) and acidattack of the combustion residue.

(C) Electrolytic disintegration of the element. It seems impossible toavoid the subsequent operation of combustion or of grinding, as it isnot possible to have the active material free of carbon, by making useonly of the tested electrolytic treatments.

(D) Volatilization process (chlorides-fluorides) of the active elements.In this case too the first step is represented by the disintegration orcombustion of most of the graphite.

{Without dwelling excessively in the enumeration of the seriousdrawbacks which all the above-mentioned processes present, it will besufficient to note that the exceptional radio-activity of this type ofelement complicates tremendously any mechanical handling.

For example, process B (combustion), which is the most studied andknown, implies in all its variants (air,

oxygen, nitrogen oxide) the production of enormous amount of highlycontaminated combination gases.

The treatment of said gases, imposes the use of various washing systemsand different types of filters, all having noticeable sizes anddifficult maintenance.

The further ejection to the atmosphere of filtered gases, even afterlarge dilution, represents also a technicaleconomic problem of seriousdifliculty.

The process of the present invention, tends to eliminate completelydrawbacks of this type.

According to the present invention the process for the transport of thecarbon may be illustrated as follows:

Let us consider a closed system; in a zone of this system which we callattack zone the temperature and pressure conditions are such as toproduce a reaction between the carbon and the fluid. therein present.

The so formed gaseous product, by simple diffusion or by circulation,however accomplished, goes to another zone of the same system, which wecall regeneration zone.

In this zone the temperature and pressure conditions are such as toproduce a reaction which is the reverse of that which took place in theattack zone. Consequently carbon will be deposited and the attack fluidwill be regenerated. In other words the system will go back to theinitial state with the only difference that the transport of carbon tookplace (and the subsequent inevitable losses of energy). Variousreversible reactions, long well known, can be differently used in oursystem to achieve the object of the present invention.

Therefore, in order to illustrate more particularly the presentinvention, some examples, i.e., a certain number of specificembodiments, are shown by way of unrestrictive example:

A first practical example of what is above-mentioned, is given by theequilibrium reactions of the system The proportion of the two gases inequilibrium (in the presence of carbon) depends only on the pressure andon the temperature. The pressure increase shifts the equilibrium to theleft (carbon is deposited) and the temperature increase shifts theequilibrium to the right (carbon is absorbed).

Therefore it is possible to adjust these two parameters in such a waythat in the attack zone, the reaction may be shifted widely to the leftand so that in the regeneration zone the contrary takes place.

As pointed out above, the only change due to the operation of the systemis represented by the transport of carbon from one zone to another andby the inherent energy losses.

A second example is given by the reversible equilibrium reactions of thesystem Also in this case the proportion of the two gases in the presenceof carbon depends on the pressure and on the temperature.

The preceding considerations are also valid, except that in this casethe pressure increase shifts the reaction to the right (absorption ofthe carbon) and the temperature increase shifts the reaction to the left(cracking).

As a matter of fact, a result similar to that of the two above-mentionedexamples, may be obtained by means of more than two reactions which maybe convenient for particular reasons, provided that at the end of thecycle, the system returns to the initial state with the exception of theoccurred transport of carbon, which remains always the useful object tobe reached.

A practical example of the third possibility is given by the followingreactions:

After separation of water by condensation and cracking of pure methane,the state of the system has gone back to the initial one and only thecarbon has been transported.

The so-called regeneration reactions in these systems are carried outmore easily and occur at a lower temperature when suitable catalysts areused.

However they are possible also without the use of said substances whenprompted by economical consideration or other reasons. The reaction 2COC+CO on finely divided nickel develops already entirely at 350 C. andmore slowly on finely divided iron at 450 C.

The reaction CO+3H =H O+CH is very active on nickel at 230-250 C.

The process, object of the present invention eliminates entirely theproblem represented by the discharge of large amounts of contaminatedgases to the atmosphere, and moreover reduces or eliminates the need offilters, as the particles of fission products which possibly pass in thezone wherein carbon is deposited, will go to waste storage.

The extraction of volatile products or fission gases, if it isnecessary, will involve small amounts of gas.

One of the methods of absorbing these substances (active volatileproducts and fission gases, for example xenon) may be that of lettingthem recirculate in vessels containing deposited carbon which is finelysubdivided (before separating said vessels) at low temperature and forthe time necessary to determine the absorption.

In this way, these fission products can be eliminated together withcarbon which goes to the discharge storage of active products. Finallythe carbon consigned to the waste storage remains closed in the vesselin which it has been deposited without undergoing other manipulationsother than the ones necessary for separating the vessel from the systemand possibly for reducing the volume by compression. The tests carriedout by us have given us quantitative data on the velocity of the variousreactions in the operating conditions. Particularly on the attack ofcarbon with CO our tests have pointed out a very rapid increase of thesample area (graphite density 1.6) in function of time. So that, foreach temperature, the curve representing the carbon removed in functionof the time increases rapidly in a non-linear way, becoming almostvertical at about the end of the operation.

This perculiarity reduces enormously the time of attack if thecombustion were preceded by grinding, whose grains should reduce itsdimensions in function of time.

FIG. 1 illustrates schematically, by way of example, an embodiment ofthe first not limitative example of our process.

In stainless steel container 4 are placed fuel elements sheathed withgraphite of the type HTGR 11 or large pieces of the same. The furnace 1keeps the container at 1000 C.

From the bottom of container 4 penetrates the CO gas driven by blower 8through filter 9. CO at temperature of 1000 C., reacts with carbonforming CO, sucked by blower 8 through pipe 5 into container 6 andcooled, deposits carbon on porous material 7 and gives rise to theformation of CO according to the reaction 2CO C+CO Blower 8 sends CO tocontainer 4 again, where the cycle begins again.

Filters, condensers and other devices, suitable to sep arate solidparticles and volatile products from the gaseous 4 fluid may beinstalled along the path of gases and before blower 8.

The active material (little spheres) which has not taken part in thereactions, will collect under the grid 10 and may be drawn from theopening 9.

In said FIG. 1, reference numeral 2 indicates the electric resistancesof the furnace, and 3 the cooling system.

A variant of the described devices is represented by the possibility ofreplacing the inert porous material 7 (FIG. 1) with a suitable catalystfor the regeneration reaction (2CO- C+CO The use of this catalyst makesthe reaction much more rapid and complete, allowing also a considerabledecrease in the operating temperature. Suitable materials for saidfunction are the iron, nickel, cobalt and others in metallic form and ina condition of very fine division.

Said variant implies also two different possibilities of using, in caseof utilization of catalysts:

(a) In one case, the carbon precipitated on the catalyst may go out fromthe circuit and may be sent to waste storage together with the metalliccatalyst which therefore is not recovered.

(b) In the second case the container of carbon precipitated togetherwith the catalyst is treated for recovery of the catalyst, before beingsent to waste storage of carbon. Said recovery according to thedimensions and other features of the reprocessing plant, may be of theelectromechanical type (magnetic separation in case of iron) and ofchemical type (iron, nickel as illustrated schematically in FIG. 2).

Conduits 3, in FIG. 2 serve to bring again the CO alternatively to thebottom 5 of the container through valve 12 suitably open in thisdirection. Conduit 6 is necessary to conduct CO produced by the reactionin vessel 1 through valve 11 suitably open in that direction which inthis phase is already full of nickel and of deposited carbon.

CO, at the temperature of C. and at a high pressure, turns the existingnickel into nickel-carbonyl Ni(CO) Said nickel-carbonyl decomposes incontainer 2, at 400 C., into Ni, 2C and CO When container 1 becomes freeof Ni, it is taken off from the system and is sent to waste storage. Thenew container is inserted in position 2 and by reversing the flux of COwhich now flows into duct 7, and by regulating the temperatures, in saidcontainer 2 there is obtained the formation of nickelcarbonyl, whichgoes to decompose into the new container inserted in position 1.

In this way, alternately, the two containers accomplish the twofunctions: (a) to deposit carbon on Ni and (b) to transport Ni into thesecond container under the form of nickel-carbonyl.

FIG. 3 shows a schematic device to illustrate the third possibility,i.e. the possibility that also with various reactions, different fromthe ones already indicated, the system is returned to the initial state,except for the removing of carbon which was the aim to be reached.

In vessel 1 containing carbon 2 to be attacked, steam arrives throughduct 3 at temperatures between 800 C. and 1000 C. It enters the reactionwith C.

H O+C=CO+H at high temperature 2H O+C=CO +2H at low temperature 3HO+2C=CO +CO+3H at intermediate temperature Hydrogen and the mixture senton finely divided nickel 6 at 250 C., are added to the so produced watergas, through pipe 4 in point 5.

In any case if H is in light excess, CO is not formed but only H 0 andCH A condenser 7 separates the water which goes to evaporator 8 to berecycled through 3, whereas methane (pure) is decomposed in a crackingtower 9 at temperatures between 400-800 C. into H and C with or withoutcatalyst. The hydrogen is recycled in 5 and carbon is sent again toWaste storage 10. Also in this case the carbon container may becompressed or deformed to reduce the volume.

Although the present invention has been described in connection with aparticular embodiment illustrated in the drawings, the inventive conceptis susceptible of numerous other applications which will occur to peopleskilled in the art.

In addition, without departing from the scope of the invention, manyvariations may be brought to the present invention, all these variationsbeing comprised in the above-mentioned main concepts.

What we claim is:

1. In the reprocessing of nuclear fuel elements which contain carbon inaddition to active material, the method of eliminating carbon therefromwhich comprises transporting said carbon from a first zone containingsaid nuclear fuel elements to a second zone in a closed system bycausing the reaction of said carbon with carbon dioxide at a temperatureof about 1000 C. to convert the same into carbon monoxide, causing saidcarbon monoxide to flow to the second zone, maintaining a reducedtemperature in the second zone so that said reaction is reversed toconvert the carbon monoxide to carbon dioxide and regenerated carbon andto deposit the regenerated carbon in said second zone, and thencausingthe carbon dioxide in said second zone to flow back to the firstzone.

2. The method of eliminating carbon from nuclear fuel elements in aclosed system as claimed in claim 1 wherein the temperature in saidsecond zone is maintained at about 600 C.

3. The method of eliminating carbon from nuclear fuel elements in aclosed system as claimed in claim 1 wherein the reaction in said secondzone is conducted in the presence of a finely divided catalyst selectedfrom the group consisting of nickel, iron and cobalt.

4. The method of eliminating carbon from a nuclear 5 fuel element in aclosed system as claimed in claim 3 wherein the catalyst is finelydivided nickel and the temperature in said second zone is maintained atabout 350 C.

5. The method of eliminating carbon from nuclear fuel elements in aclosed system as claimed in claim 3 wherein the catalyst is finelydivided iron and the temperature in said second zone is maintained atabout 450 C.

6. The method of eliminating carbon from nuclear fuel elements in aclosed system as claimed in claim 1 wherein the carbon regenerated inthe second zone is deposited in a removable container.

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25 2,464,532 3/1949 Sellers 252 417 X 2,758,098 8/1956 Haensel 2524l63,260,574 7/1966 Hatch et a1 23 324 3,453,091 7/1969 Knotik et a1. 23324 EDWARD J. MEROS, Primary Examiner US. 01. xn.

